Sewage sludge is generated during the treatment of wastewater in wastewater treatment facilities, septic tanks, and lagoons. By definition, sewage sludge is “solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works.” (U.S. Environmental Protection Agency (EPA) regulations in Title 40 of the Code of Federal Regulations (CFR), Part 503.9(w)). Sewage sludge typically contains harmful microorganisms, such as pathogenic bacteria, enteric viruses, and helminth ova. In addition, sewage sludge typically produces unpleasant odors due to bacterial breakdown of fat, protein, and carbohydrate molecules into volatile molecules. Also, sewage sludge often attracts undesirable pests, such as flies and rats, a problem which is referred to as “vector attraction.”
Sewage sludge that has a relatively high water content is typically dewatered at some point in its treatment. The reduction of the water content of the sewage sludge makes it easier to handle, among other things. After dewatering, the sewage sludge typically is in the form of an 18-30% solids cake, with the balance of the weight of the sludge being water. Before the sewage sludge can be applied to land without restriction, it must meet pathogen reduction requirements. Several alternatives exist to achieve pathogen reduction, including composting, heat treatment at >160° F., incineration, or treatment with chemicals such as methyldithiocarbamic acid or its salts.
In some cases additional treatment of the sewage sludge may be needed to reduce vector attraction. One way of doing this involves treating the sewage sludge with an agent to increase its pH to 12 or higher. Sodium hydroxide and potassium hydroxide are two compounds that are commonly used for this purpose. Although this method is effective to reduce vector attraction, it also has the undesirable effect of changing the consistency of the sewage sludge from a moist, friable solid to a slimy semi-liquid. In this condition, the sewage sludge is difficult to handle, and does not respond to conventional conditioning methods such as flocculant addition, heat treatment, or filtration.
There is a need for improved processes for treating sewage sludge that can overcome or minimize one or more of the problems discussed above.